1. Field of the Invention
The present invention relates to a magnetic recording medium and a magnetic storage apparatus. More particularly, it relates to a technology of a longitudinal magnetic recording medium which has a low noise and a high coercivity, and is also sufficiently stable against thermal fluctuation. Further, it relates to a technology of a high reliability magnetic storage apparatus having an areal recording density of 50 megabits per square millimeter or more, which has been implemented by combining the longitudinal magnetic recording medium technology with a technology of a high sensitivity magnetic head, and optimizing the read/write conditions.
2. Description of the Related Art
In recent years, there is an increasingly growing demand for an improvement in areal recording density of a magnetic recording medium with an increase in capacity of a magnetic recording disk drive. A reduction in media noise is indispensable for improving the areal recording density. To that end, the grain size of a magnetic layer is required to be made fine for increasing the number of grains per bit. However, microfine magnetic crystal grains show a tendency to generate magnetization reversal due to the influence of thermal fluctuation. Accordingly, the decay of recorded magnetization, that is, the thermomagnetic relaxation phenomenon becomes noticeable. In order to suppress the thermomagnetic relaxation phenomenon, the thermal stability factor (Kuxc2x7v/kT) is required to be kept at generally 80 to 90 or more, wherein Ku is the crystal magnetic anisotropy constant, v is the volume of a magnetic crystal grain, k is the Boltzmann constant, and T is the absolute temperature. When the magnetic crystal grain has been made fine, the grain volume v is reduced. Accordingly, Ku is required to be raised for keeping Kuxc2x7v/kT at not less than the aforesaid value. If Ku is improved, the anisotropy field (Hk) is also increased. If the Hk exceeds the recording magnetic field from a magnetic head, however, the overwrite characteristic is largely deteriorated. For this reason, the Hk of the medium is required to be set so as not to exceed the recording magnetic field from the head. This requirement decides the upper limit on the Ku value.
As a technology for ensuring compatibility between the suppression of thermomagnetic relaxation and a reduction in noise, an anti-ferromagnetically coupled medium has been proposed in recent years (Appl. Phys. Lett., vol. 77, pp. 2581-2583, October (2000), and Appl. Phys. Lett., vol. 77, pp. 3806-3808, December (2000)). This medium is so configured that a magnetic layer portion has a double-layered structure in which respective magnetic layers are anti-ferromagnetically coupled via a Ru intermediate layer therebetween. In the anti-ferromagnetically coupled medium, the magnetization of a magnetic layer (lower magnetic layer) on the substrate side and the magnetization of another magnetic layer (upper magnetic layer) on the protective layer side are oriented in antiparallel to each other in the residual magnetization state. For this reason, when the product (Brxc2x7t) of the residual magnetic flux density (Br) and the magnetic layer thickness (t) is made equal to that of the medium using a magnetic layer of a single-layered structure, it is possible to increase the thickness of the upper magnetic layer which is a recording layer. Thus, it is possible to raise the thermal stability factor (Kuxc2x7v/kT) of the magnetic layer.
However, the foregoing technology falls short of specifically providing a longitudinal magnetic recording medium which has a low noise and a high coercivity, and is also sufficiently stable against thermal fluctuation.
It is an object of the present invention to provide a longitudinal magnetic recording medium which has a low noise and a high coercivity, and is also sufficiently stable against thermal fluctuation. In addition, it is another object to provide a high reliability magnetic storage apparatus having an areal recording density of 50 megabits per square millimeter or more by combining the longitudinal magnetic recording medium with a high sensitivity magnetic head, and optimizing the read/write conditions.
In order to achieve the foregoing objects, in accordance with the present invention, the longitudinal magnetic recording medium has been so configured that a magnetic layer is formed on a non-magnetic substrate via a plurality of underlayers, wherein the magnetic layer is made up of a lower magnetic layer containing at least one of Ru or Re in an amount of not less than 3 at % to not more than 30 at %, and Cr in an amount of not less than 0 at % to not more than 18 at %, and further containing at least one of B or C in an amount of not less than 0 at % to not more than 20 at %, and the balance being made up of Co, and an upper magnetic layer containing Co as a main component disposed thereon via a non-magnetic intermediate layer.
Further, the plurality of the underlayers has been so configured as to include a non-magnetic and amorphous structured first underlayer containing Co or Ni as a main component, and a body-centered cubic structured second underlayer containing Cr.
Further, a B2 structured alloy material may also be used for the first underlayer.
Still further, at least one layer of the plurality of the underlayers has been so configured as to be made of a non-magnetic and hexagonal close-packed structured alloy material containing Co.
Furthermore, it is also possible that the Co-containing alloy underlayer is formed as a third underlayer between the lower magnetic layer and the second underlayer.
Further, the underlayer made of a non-magnetic and hexagonal close-packed structured alloy material containing Co has been so configured as to be made of a Coxe2x80x94Ru alloy containing Ru in an amount of not less than 35 at % to not more than 60 at %.
Still further, at least one layer of the plurality of the underlayers have been so configured as to be made of a Cr-containing body-centered cubic structured alloy material, wherein the Cr alloy contains B in an amount of not less than 2 at % to not more than 15 at %.
Furthermore, in a magnetic storage apparatus having: a magnetic recording medium; a driver for driving it in the recording direction; a composite head having an inductive magnetic head for recording and a spin-valve type magnetic head for reading in combination; a means for causing the head to perform relative movement with respect to the medium; and a read/write signal processing means with respect to the head, the magnetic recording medium has been allowed to be configured with the longitudinal magnetic recording medium.